Protective coatings are of two types. Protective coatings consist of either conversion coatings or barrier type coatings. Conversion coatings involve a chemical reaction that modifies a substrate surface. Typical conversion coatings utilize chromate treatments on metals or alloys such as aluminum, steel, and galvanized steel. Metal surfaces are normally coated from an aqueous solution that contains hexavalent or trivalent chromium ions, phosphate ions and/or fluoride ions. There is an increased environmental concern over the use of chromate (chromium) anti-corrosion treatments because of the leaching of toxic chromium salts into the environment.
A conversion coating typically modifies the composition or microstructure of the surface of the substrate by means of a chemical reaction or treatment. Such treatments usually result in producing a modification to the surface morphology. Examples include pack cementation and slurry cementation, specifically chromating and aluminizing. These techniques utilize diffusion to produce a surface coating that diffuses into the substrate. Thus a composition gradient exists between the surface of the treated substrate and the interior.
Other techniques involve the use of protective ceramic coatings or coatings of organic resins. Coatings comprised of organic resins typically function as barrier coatings. Barrier type coatings overlay a substrate and protect it from erosion; corrosion and in some cases strengthen a substrate. Despite considerable efforts these have not provided the equivalent corrosion resistance that chromate or aluminized based coatings provide.